This invention is directed to a crossed-field switch device and an ignition method therefor whereby the crossed-field switch device can be onswitched with voltage applied without pulsing the magnetic field to a high value.
In recent years crossed-field switch devices have been developed into tubes which are capable of conducting fairly high currents and are capable of offswitching against high voltages. Crossed-field switch devices having a 10,000 ampere DC conducting capability and an offswitching capability against 100 kilovolts have been designed. Such switch devices are believed to have a considerable prospect in the developing field of high power electric transmission by means of direct current links. Such crossed-field switch devices do not have long term conducting capability, and thus must be paralleled during normal conduction. When it is desired to open the circuit, the in-line switch is opened, so that it shunts the current through the crossed-field switch device which is thereupon turned off. U.S. Pat. No. Re. 27,557 illustrates this type of circuit breaker which incorporates a crossed-field switch device as an off-switching component.
There have been a number of developments in the art of the crossed-field switch device which have brought it to this state of utility. Among the background patents on the crossed-field switch devices are U.S. Pat. Nos. 3,638,061; 3,641,384; 3,604,977; 3,558,960; 3,678,289; 3,769,537 and 3,749,978.
Of course it is necessary to onswitch the crossed-field switch device when current flow therethrough is required. Under the right conditions of applied voltage and magnetic field, initial ionization can come about by the action of cosmic rays. However, in order to reduce the satistical reliance on such events, ignition devices can supply the preionization, (see U.S. Pat. Nos. 3,714,510 and 3,890,520). These patents covering ignition equipment are useful to reduce the ignition time delay whenever the conditions are within the conductive region of the Paschen curve.
Sometimes it is desired to make the crossed-field switch device conductive, that is onswitch the switch device, when the rated line voltage is applied thereacross. This has formerly been done by pulsing the magnetic field sufficiently high that even with voltage applied, the conditions in the interelectrode space move into the conductive region. For example see U.S. Pat. Nos. 3,678,289 and 3,604,977.
However, the firing or ignition of crossed-field switch device at high voltage using a high pulse magnetic field has several disadvantages. There is a time delay from the trigger to the ignition of the crossed-field switch device in the order of 10 microseconds. Furthermore, there is significant jitter in ignition, in the order of 1 microsecond. Additionally, it is difficult to obtain short duration, high level magnetic pulses in the order of 0.1 Tesla or 1 K Gauss in crossed-field switch devices due to the eddy currents created in the electrodes. Also, a high powered magnetic field pulser is a difficult and expensive device, particularly at a high pulse repetition frequency. These requirements also create a more complicated and expensive tube construction which often includes an internal magnetic field coil to minimize the above listed detrimental effects. Thus, it is highly desirable for a crossed-field switch device to be ignited at high voltage, in the order of 10 to 100 kilovolts, with a relatively low magnetic field, in the order of 0.01 Tesla, or 100 Gauss.